Combine development in recent years have put greater demands on the straw choppers and chaff spreaders. The main reasons are that larger combines and wider cutting widths are creating higher flows of straw and chaff; wider cutting widths create more demanding spreading widths; reduced tilling practices have put more demand on even spreading of both straw and chaff and that plant breeding has resulted in tougher straw, higher yields and more residue.
Large capacity combines have remained the same physical size as previous models. This means that a greater flows of grain, chaff and straw are moving through the same sized cavities. For this reason, greater mass flows of air are required to separate the chaff and grain on the sieves and in advanced straw chopping systems, a greater volume of air is being pressurized to spread the residue wider. This means that there must be a large exhaust/inlet area to balance the airflow so that the combine hood will not pressurize and effect grain separation.
As combines get larger and more compact the distance between the end of the sieve and the straw chopper is shrinking as this space is sacrificed for other options. The space between the chopper and the sieve end typically is the area where air pressure is balanced. Reduction of and enclosing of this space has the potential to create problems with the airflow off of the sieves and therefore alter the grain sample—leaving poorly separated grain or a dirty sample.
Typically the airflow off the sieves carries the chaff and light partials into a chaff spinner or straw chopper. The airflow trajectory needs to remain substantially rearward so it will not create vortices or non laminar flow across the sieves with the potential of creating a dirty sample.
The current John Deere STS combine is perhaps the most extreme example of a compact machine. In this case Deere has added a small opening in the sidewall of the combine, nearly perpendicular to the end of the sieves and substantially immediately at the end of the sieves. Although in one condition this does relieve some air pressure, the placement of the opening interferes with the proper operation of the sieves in that the air from the sieves must make a substantially immediate 90-degree turn as it comes off of the sieves. This leads to vortices on the sieves and ultimately to areas of the sieves that contribute to a poor grain sample. Inversely, when the chopper is demanding more air than is being supplied by the sieve fan the air must be drawn in from a location near the sieves which again leads to vortices on the sieves and ultimately to areas of the sieves that contribute to a poor grain sample.
At this time no other solution is available which will allow the large amount of air exiting/drawn in the rear of the sieves to escape/enter the combine.
U.S. Pat. No. 4,412,549 (Rowland Hill) issued November 1983 and U.S. Pat. No. 6,558,252 (Visagie) issued May 2003 both disclose air flow arrangements which improve separation at the sieve in the combine harvester.